Photovoltaic energy is becoming a very significant source of electrical power. This is because problems of scarcity and safety have limited the use of fossil and nuclear fuels, and recent advances in photovoltaic technology have made possible the large scale manufacture of low cost, light weight, thin film photovoltaic devices. It is now possible to manufacture large scale, thin film silicon and/or germanium alloy materials which manifest electrical and optical properties equivalent, and in many instances superior to, their single crystal counterparts. These alloys can be economically deposited at high speed over relatively large areas and in a variety of device configurations, and as such they readily lend themselves to the manufacture of low cost, large area photovoltaic devices. U.S. Pat. Nos. 4,226,898 and 4,217,364 both disclose particular thin film alloys having utility in the manufacture of photovoltaic devices of the type which may be employed in the present invention. However, it is to be understood that the present invention is not limited to any particular class of photovoltaic materials and may be practiced with a variety of semiconductor materials including crystalline, polycrystalline, microcrystalline and noncrystalline materials.
The power generated by a photovoltaic device is proportional to the illumination incident thereupon and if relatively large amounts of power are to be generated, fairly large collection areas are required. The roof and upper story areas of building structures are well illuminated and are generally not put to productive use. For some time now it has been known to place photothermal and photovoltaic collectors on the top portions of buildings. Roof mounted photovoltaic devices are shown for example in U.S. Pat. Nos. 5,092,939; 5,232,518 and 4,189,881. These particular photovoltaic roofing structures are of the batten and seam type.
In many instances shingled roofs are favored, typically for residential construction, and in those instances where fairly complex roof geometries are encountered. In a typical shingle construction, roofing material is supplied in rolls, or in precut pieces which are subsequently laid in an overlapping configuration. In some instances, roofs are shingled with relatively thick tiles, which may be planar or of a curved cross-section. It will be appreciated that there is a need for integrating photovoltaic power generation with shingled roof constructions.
U.S. Pat. No. 4,040,867 describes a photovoltaic shingle construction comprised of a plurality of individual shingle members, each of which has a number of electrically interconnected single crystal photovoltaic devices thereupon. In order to obtain high power from this type of device, either the individual shingle must be made larger, or several shingles need to be electrically interconnected. The first approach presents problems of windloading; and the second approach results in a construction requiring a large number of weatherproof electrical interconnections; also, leakage can result because of moisture creep between adjacent shingles by capillary action. Another configuration of photovoltaic shingle is described in U.S. Pat. No. 4,321,416. U.S. Pat. No. 3,769,091 depicts yet another photovoltaic roofing system comprised of a number of individual silicon devices mounted in an overlapping relationship.
The prior art has not been able to provide an acceptable shingle type photovoltaic roofing system. Prior art devices are generally thick, inflexible, or of a geometry which makes them incompatible with standard construction techniques. As a result, prior art photovoltaic shingle structures require specialized installation techniques and trained personnel, which increases their cost and limits their utility. Furthermore, such structures cannot be easily integrated into a conventionally constructed roof. In addition, prior art photovoltaic roofing structures present aesthetic problems since the devices are often of a distinctive color, or of a geometry such that they are very obvious when installed.
Clearly, it would be desirable to have a photovoltaic roofing material which is as much like conventional roofing material as possible. The photovoltaic portion of the roofing material should be self contained to a large degree and be easily installed by conventional techniques. It should also be relatively lightweight, resistant to wind loading and stable under harsh atmospheric conditions.
The present invention, as will be described in further detail herein below, provides a roofing material which incorporates photovoltaic technology into conventionally configured shingle stock. The roofing material of the present invention is simple to install and efficiently converts light to electricity, and may be used in combination with standard, non-photovoltaic shingle stock to cover any desired portion of a roof. The particular configuration of the present invention makes efficient use of roof space for generating electricity and is unobtrusive in use. These and other advantages of the present invention will be readily apparent from the drawings, discussion and description which follow.